1. Field of the Invention
The present invention relates to an image reading device, and more particularly, to an image reading device having a one-dimensional line sensor for reading the image information of an original maintained in contact with said line sensor and moved with respect thereto, and adapted for use in a facsimile apparatus, an image reader or the like.
2. Related Background Art
In the field of so-called contact-type image reading devices, there has been principally employed a structure incorporating a rod lens array for projecting the image of the original onto a photosensor or photosensors having phtoelectric converting elements.
On the other hand, there has recently been developed image reading devices which do not use the rod lens array for the purpose of cost reduction and compactization, but have a thin transparent protective layer formed on the photosensor, for direct contact with the original during image reading.
FIG. 1 illustrates an example of the schematic construction of such an image reading device, and shows an original pressing roller 1; an original 2; a translucent substrate 3 having unrepresented photoelectric converting elements on the surface thereof; a translucent protective layer 4 adhered onto the surface of the substrate 3; a structure 5 for supporting the substrate 3; and a light source 6 for illuminating the original.
In such an image reading device, the light from said light source 6 enters from the bottom face of the translucent substrate 3, and reaches the original 2, passing through the vicinity of the photoelectric converting elements formed on the surface of the substrate 3, and the light diffused on the surface of the original 2 enters the photoelectric converting elements through the translucent protective layer 4.
FIGS. 2 and 3 illustrate an example of the pattern of the photoelectric converting elements formed on the surface of the translucent substrate 3, wherein FIG. 2 shows an end portion of an array of the photoelectric converting elements while FIG. 3 shows a central portion of said array.
Referring to FIGS. 2 and 3, there are shown photoelectric converting elements 7; capacitors 8; thin film transistors (TFT) 9; TFT driving signal lines 10; and reading signal lines 11, wherein the components 7 to 11 are opaque to light, while the remaining substrate areas are translucent.
If the illuminating light source 6 is sufficiently long in the direction of the array of the photoelectric converting elements and has uniform light intensity thereby giving uniform light intensity on the bottom face of the translucent substrate 3, uniform outputs can be expected from the photoelectric converting elements as shown in the FIG. 4, in case of reading an original of uniform density. However, an unevenness in the output, for example, as shown in FIG. 5 occurs in most of the image reading devices of a pattern as shown in FIGS. 2 and 3. Such unevenness results from the variation in the amount of light reaching the original 2 through the translucent areas, and from the variation in the amount and form of the light beam entering the photoelectric converting elements 7 after being diffused on the original, since the translucent and opaque patterns in the vicinity of the photoelectric converting elements 7 slightly vary from location to location. Due to the occurrence of such unevenness, the S/N ratio of the image signal is deteriorated, thereby the image quality is poor.